Uplink Beam Forming
Currently, the Third Generation Partnership Project (3GPP) is evaluating the potential benefits of uplink transmit diversity in the context of High-Speed Uplink Packet Access (HSUPA) (see 3GPP Tdoc RP-090987, 3GPP Work Item Description: Uplink Tx Diversity for HSPA). With uplink transmit diversity, UEs that are equipped with two or more transmit antennas are capable of utilizing all of them. This may be achieved by multiplying the transmit signal s(t) with a weight factor W=[w1, w2, . . . , wi, . . . , wM]. The term weight factor can also be referred to as a precoding vector. Referring to FIG. 1, which shows a schematic diagram of uplink transmit diversity. Note that i=1 . . . M, where M denotes the number of transmit antennas. The rationale behind uplink transmit diversity is to adapt weights so that the user and network performance can be maximized in case of beam forming where the UE can transmit from more than one antenna simultaneously at a given time-instance.
According to whether there is an explicit feedback from a Node B, the uplink beam-forming can be divided into two types:                Open Loop Beam Forming (OLBF): the UE autonomously decides the antenna weights. The UE selects the precoding vector with the assist of the existing feedback from the Node B, such as Uplink Transmit Power Control (UL TPC), Hybrid Automatic Repeat Request (HARQ) feedback, etc.        Close Loop Beam Forming (CLBF): the Node-B provides an explicit feedback to the UE stating which weights the UE should use when transmitting the signal.        
The study on Open Loop Transmit Diversity (OLTD) was disclosed in 3GPP TR 25.863, Uplink transmit diversity for High Speed Packet Access (HSPA) (Release 10). For a UE in an OLBF mode, the uplink beam direction is adjusted by the UE based on the received uplink TPCs.
When the UE is in soft handover, uplink TPC commands from all connected Node Bs in a slot are combined according to the following policy: the combined TPC is UP if the TPCs received from all connected Node Bs are UP and the combined TPC is DOWN if any of the TPCs received from the connected Node Bs is DOWN.
The best connected Node B in uplink for a UE in soft handover has higher opportunity to generate uplink TPC DOWN than other connected Node Bs. Hence, the best connected Node B is dominant in TPC combining according to this TPC combining policy. The UE in OLBF mode does channel sounding by adjusting the uplink beam direction in opposite directions and compares the uplink TPCs corresponding to the two opposite directions. If a TPC DOWN is received in one direction and a TPC UP is received in the opposite direction, then the UE turns the uplink beam towards the former direction. As the best cell is dominant in TPC combining for a UE in soft handover, the UE turns the uplink beam towards the best connected Node B gradually. If the best Node B in uplink changes, the uplink beam changes gradually towards the new best Node B. When the uplink beam direction adjustment is done at a side of the UE in the OLBF mode, the network is not notified.
Currently, the investigation of Close Loop Transmit Diversity (CLTD) was started and driven by the main vendors of wireless communication systems in 3GPP. In case of CLBF, it is not decided whether only a serving cell (or a Node B) determines a precoding vector or non-serving cells can also generate and determine the precoding vector. In order to avoid RoT oscillation due to frequent uplink beam direction change from one active cell to another, it is better that a servicing Node B rather than a non-serving Node B generates and determines the precoding vector. It is also possible that a non-serving cell (or a Node B) generates and determines the precoding vector for some other purpose. The uplink beams of a UE in CLBF mode direct to the active cell that generates and determines the precoding vector.
Hereinafter, a CLBF/OLBF UE means a UE in CLBF/OLBF mode rather than a CLBF/OLBF capable UE only, and the cell to generate and determine the precoding vector for a UE in uplink beam forming (BF) mode is referred to as BF-control cell for this UE. For an OLBF UE, a cell having the best uplink quality is usually a BF-control cell.
Uplink Beam Direction Change During Handover
When a BF-control cell of a CLBF UE changes from one cell to another, the uplink beam of the CLBF UE turns from towards the current BF-control cell towards the target BF-control cell as well. Refer to FIG. 2, which shows an example of uplink beam direction change. As shown in FIG. 2, Cell A and Cell B are two active cells of a UE. Before uplink beam direction of the CLBF UE is changed, Cell A is the BF-control cell of the UE and experiences interference from the main lobe while Cell B is one non-BF-control cell and experiences interference from a weaker lobe. After the beam is changed, Cell B turns to be the BF-control cell and experiences interference from the main lobe while Cell A turns to be one non-BF-control cell and experiences interference from a weaker lobe. When the beam changes from Cell A to Cell B, the uplink load of Cell A due to this CLBF UE is suddenly decreased but the uplink load of Cell B due to this CLBF UE is suddenly increased.
The uplink beam direction change for an OLBF UE is not clear for the network because the network does not explicitly control precoding of uplink beams. Dependent on algorithms for selecting the precoding vector in the UE and fading changes, the required time of BF-control cell change can be quite different. If the uplink beam direction changes too fast for an OLBF UE, there can be uplink stability problem for a cell connected with the OLBF UE.
The beam direction changes as the BF-control cell changes. The beam direction change is unavoidable and can be very often due to mobility of users.
Uplink Load Estimation
The uplink load of a UE can be estimated based on CIR of DPCCH. Suppose a UE has N parallel uplink channels, the total uplink load that a UE generates can be estimated based on DPCCH CIR as (see “WCDMA for UMTS—Radio Access For Third Generation Mobile Communications”, third Edition, Harri Holma, Antti Toskala, the disclosure of which is incorporated herein in its entirety by reference):
                              Load          =                                                    CIR                DPCCH                            ·                              (                                  1                  +                                                            ∑                                              i                        =                        1                                                                    N                        -                        1                                                              ⁢                                                                                  ⁢                                          pwroff                      i                                                                      )                                                    Antgain              +                                                (                                      1                    -                    orthogonality                                    )                                ·                                  CIR                  DPCCH                                ·                                  (                                      1                    +                                                                  ∑                                                  i                          =                          1                                                                          N                          -                          1                                                                    ⁢                                                                                          ⁢                                              pwroff                        i                                                                              )                                                                    ,                            (        1        )            where CIRDPCCH is a (estimated) DPCCH CIR and can be either a target DPCCH CIR or a measured DPCCH CIR; Pwroffi is a power offset of the ith channel with respect to DPCCH; Antgain is an (estimated) antenna gain; and orthogonality is a (estimated) channel orthogonality.
For uplink beam-forming users, when a BF-control cell for an uplink beam-forming UE changes, the beam direction changes from the current BF-control cell to the target BF-control cell can result in sudden uplink load decrease in the current BF-control cell and sudden uplink load increase in the new BF-control cell, which further causes the RoT oscillation in both current BF-control cell and target BF-control cell. Take FIG. 2 as an example. Before the uplink beam direction change (i.e., BF-control cell change), Cell A is a BF-control cell and Cell B is a non-BF-control cell, and uplink loads generated by UEs in Cell A and Cell B are LA,0 and LB,0, respectively. After the uplink beam direction change, Cell A turns to be a non-BF-control cell while Cell B turns to be the BF-control cell, and uplink loads generated in Cell A and Cell B are LA,1 and LB,1, respectively. Because uplink beam forming is used, the load generated by the UE in Cell A is decreased (LA,1<LA,0) and the load generated by the UE in Cell B is increased (LB,1> LB,0) if the uplink load grant of UE is not changed during BF-control cell change. According to 3GPP protocols, an uplink load grant is the maximum power offset of the load, which can be used by the user, with respect to the uplink DPCCH power, and an uplink load grant for a user determines the maximum number of bits in uplink transport data blocks for the user. One active cell of an UE may evaluate using such a parameter the maximum allowed uplink load generated by the UE in the cell.
This RoT oscillation in the cell is serious when one or more of the following conditions are satisfied:                The target BF-control cell already has a high uplink load        The uplink beam-forming UE is causing a high uplink load        The UE has good uplink beam forming capability        
The similar problem also exists when the BF mode is activated for a UE from the other modes (e.g., a default mode). The uplink load of the UE in the BF-control cell can also increase due to the main lobe directs to this cell compared to without uplink beam-forming.
Due to the change of the UE beam direction, the caused RoT peak in the new BF-control cell (i.e., the target BF-control cell) can exceed the RoT target quite much and triggers congestion actions, which deteriorates the uplink performance.